Why you should do this exercise: It's a fun and challenging exercise. Even if you have access to a fully stocked gym, the push-up will provide a very functional compound movement by working several muscle groups simultaneously ­ and you'll be doing it all using your own body weight and mechanics.

The average person exercises their legs and lower-body muscles more than they do their upper-body muscles. This is often a result of indirect exercise such as walking, stair climbing and standing. Their upper-body muscles are at a disadvantage and they will benefit greatly by including a direct exercise like push-ups in their training regimen. Not only will this exercise stimulate the chest muscles, it will also work the abs, arms, shoulders and even the back, legs and hips.

What you need (equipment): You can certainly do push-ups using your body weight alone. For a more challenging exercise, have someone apply pressure with their hands to your upper back.

How to do it: Position your body face down on the floor. You're going to balance your body weight on your toes and hands. Your body should be straight as a board from shoulder to ankle. Begin with your arms fully extended. They should be as straight as possible without locking the elbows, with palms flat on the floor, and fingertips pointing straight ahead. Your hands should be about shoulder width apart, although you may vary hand placement ­ closer together for accentuating the triceps and wider apart for more pectoral stimulation. Look up slightly and keep your eyes fixed on a spot about 3 feet in front of you. If you have a weak lower back and find it difficult to maintain a straight body, let your butt rise up a bit so that your hips have a slight bend.

Contract and tighten every muscle from shoulders to ankles. Your body should be completely rigid. Maintain this rigid state throughout the movement to avoid stressing the lower back.

Inhale and bend at the elbows to lower yourself to the floor, keeping your elbows pointed out to the sides. Descend slowly and in control in order to work the muscles harder and give them a good pump. Stop descending when you've touched your chest to the floor ­ but DO NOT rest on the floor.

Second option: If you've never done push-ups and are concerned about doing them properly, try this modified version. Start with your knees on the floor and raise your feet up behind you. Your knees now take the place of your toes, making the exercise much more manageable.

How often should you do it? Push-ups can be done two to three times per week, depending on your overall program and goals. Doing the same exercise every day hinders gains because muscles don't actually grow or strengthen during exercise, but during the rest and recovery process that follows.

How many repetitions should you do? If the goal is purely to build strength, doing four to six reps is best. If you can't do that with straight legs, use the knee-based alternative. If you can do many more than six reps, making the exercise tougher (by having someone press down on your back) will keep you in your rep range. Sometimes people aren't strong enough to provide adequate resistance by simply pressing down on a person's back and it's difficult to gauge constant pressure for an entire set. Solution: Have a qualified person “load” a weight plate or two or three on to the upper back for increased resistance. This lower rep range provides the perfect prescription for building up your upper-body strength. For the beginner, or for general muscular growth and overall fitness, more reps are fine. Three to five sets are plenty. Doing too many reps (more than 15 or 20) is less effective for building raw strength. If you can do more than 20 reps and can't add resistance, try shortening the time between sets or slowing down the lowering portion of the movement (e.g., count to 10 as you slowly lower your body on each rep).

How you know you're doing it right? In a nutshell, you should feel the intended muscles working. By working we mean the chest muscles should become exhausted as you progress to your target repetition range. Your chest muscles will likely become engorged with blood (the muscle “pump”) and will look and feel fuller. You may also feel the chest muscles burning from a buildup of lactic acid.

How NOT to do it: Although the push-up has been around for decades, many people still don't do it correctly. Watch your butt ­ it shouldn't lie too low, and it mustn't be raised too high.

Who should do this exercise: Anyone who's interested in an exercise that hits a number of muscle groups to build strength in the upper body.

Who should NOT do this exercise: Anyone who suffers from a shoulder impingement. If you have never done the push-up, begin with the modified knee version. If you have lower-back problems, make sure not to arch your lower back.

Most common mistakes: Allowing the torso to drop too low or sticking the butt too high in the air. Doing so will take the emphasis off of the chest muscles. Placing the arms too wide. Using a ballistic jerking movement to cheat-push the body up.

This exercise is for: Almost everyone, as its intensity can be easily customized.

Charles Platkin, Ph.D., is a nutrition and public health advocate and founder of DietDetective.com.